This project addresses a basic research priority of the NEI, how do adaptational changes in retinal networks affect visual processing. The retina responds to light intensities that vary over a billion-fold, but individual retinal neurons can onl vary their output 100-fold. To overcome this disparity between input and output, retinal neurons adapt to background illumination by shifting their response ranges to span ambient illumination intensities. Adaptation occurs within photoreceptors and also at post-receptor locations in the retina. In this proposal we will study two forms of post-receptor adaptation. Using electrophysiological, pharmacological and anatomical approaches, we will investigate the retinal circuits that 1) control the transition from rod to cone signaling and 2) control the variation of center-surround spatial processing at different ambient light levels. A critical feature of the transition from rod- to cone-mediated vision is to suppress rod signaling and enhance cone signaling. Despite extensive study, the circuits responsible for this transition are poorly understood. We discovered a novel form of inhibition that suppresses the function of a critical signaling element, the rod bipolar cell, when it is activated by bright light. In aim 1, we determie whether this novel form of inhibition in rod bipolar cells contributes to the transition from rod t cone vision. Changes in the level of ambient illumination produce post-receptor adaptation that alters center-surround spatial processing. This form of spatial processing is critical for the detection of edges and spatial contrast. The circuits that mediate this form of post-receptor adaptation are unknown. In aim 2, we determine the circuits and synaptic mechanisms that produce the adaptational changes in center-surround spatial processing caused by different ambient light levels.